Update .readthedocs.yaml with required sphinx.configuration key

[lbianchi-lbl]

Summary/Motivation:

https://about.readthedocs.com/blog/2024/12/deprecate-config-files-without-sphinx-or-mkdocs-config/

Legal Acknowledgement

By contributing to this software project, I agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the license terms described in the LICENSE.txt file at the top level of this directory.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.

[lbianchi-lbl]

FYI, the CI failures are unrelated to the issue being fixed, which only affected the ReadTheDocs check.

[andrewlee94]

This seems simple enough, so I'll trust that @lbianchi-lbl knows what he is doing.